The goal of the proposed studies is to examine the time course of changes in identified cutaneous sensory neurons and spinal cord neurons responsible for the development and maintenance of inflammatory and neuropathic pain states. These neurons play critical roles in processing of pain information. These studies will address three specific aims. In the first aim we will employ combined physiological and anatomical techniques to determine the properties of primary afferents and superficial dorsal horn cells in normal mice. In the second aim we will determine the properties of the same cells after inflammatory and neuropathic injury. Behavioral tests of mechanical and heat sensitivity will be compared with any changes seen in the identified neurons. In the third aim, we will use two trangenic mouse lines (one that has the capsaicin/heat channel TRPV1 knocked out, one that has the Mas-gene related G-protein coupled receptor D (Mrgdprd) knocked out and green fluorescent protein knocked in). TRPV1 has been shown to mediate some aspects of heat sensitivity, and Mrgprd is found exclusively in a population of cutaneous sensory nociceptors. We have recently found that these cutaneous nociceptors in Mrgprd-knock-out mice, have attenuated heat responses. We will determine the properties of primary afferent and secondary nociceptors in these two mouse lines following inflammatory and neuropathic injury. By comparing behavioral and neural data, we will be able to assess the effect of these genetic manipulations on the induction and maintenance of pain states following two types of injury. The data gathered in all three specific aims will be used to develop a model of neural circuits that are essential during these pain states. By comparing the three groups, we hope to elucidate the specific populations of primary afferent and secondary nociceptive neurons crucial to this process.